The O in COPD means airway Obstruction. Obstruction means narrow airways. Narrow airways make it difficult to exhale quickly. Adults with healthy lungs can take a deep breath and then quickly exhale most of the air in their lungs within three seconds. People with asthma or COPD take 6 to 20 seconds to exhale completely. (The air remaining in the lungs after a complete exhalation is not surprisingly called the residual volume.)
There are 3 ways in which airways become obstructed: 1) bronchospasm, 2) inflammation, or 3) destruction. The first two causes can be reversed (spontaneously or successfully treated with medications). COPD is an umbrella term which includes a variable mix of all three. Asthma includes only the first two.
Emphysema is permanent destruction of small airways and alveoli. Severe emphysema is seen on simple chest x-rays or lung CT scans as blebs and bullae (small and large areas without lung tissue but full of residual air). Lung volume reduction surgery (aka LVRS) is used to remove localized bullae which are compressing normal lung tissue. This may also be done with multiple one-way valves. A low DLCO from a good quality PFT is often due to emphysema.
Okay, now for the good news. One of my colleagues at the Mayo Clinic in Rochester, Minnesota recently said that "everything is ‘COPD’ until the correct diagnosis is made." One of my mentors in Denver, Colorado (Tom Petty, RIP) told us to "look for hidden asthma" in adult smokers. What they were saying is that when airway obstruction is first noted with spirometry in current or former smokers with a chronic cough and/or shortness of breath with exertion, several other common causes for these symptoms should be thoroughly explored.
Radiologists too commonly interpret hyperinflation (relatively flat diaphragms) of the lungs as "emphysema." Hyperinflation is caused by both asthma and COPD. It is due to an excess of residual air volume (a high RV and high FRC on pulmonary function test results). Hyperinflation is reduced as airway obstruction is successfully treated.